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Transistor Digital Circuits

Transistor – voltage-controlled current source

n-type p-type

•n -channelMOSFET

•npn BJT

•p-channel MOSFET

• pnp BJT

Non-linear controlled sources: square - MOSFET

exponential – BJT

Transistor - operating principle and regionsa brief remembrance

Treshold voltage

Using Tn transistor

Output quantities: IO, VO

transfer characteristics:

IO(VCT), VO(VCT)

IT = IO

Transfer Characteristics, Tn

• VCT<VThn, Tn – off, IO=IT=0

• VCT>VThn, Tn – on, IO=IT>0

VPS=RIO+VO ; VO= VPS- RIO

VCT , IO , VO

VO,min=0 R

VI PS

Oex

• Two extreme regions, passive regions:

- cutoff (off) IO=0; VO>0; ideal switch in off state

- extreme conduction (exc) IO=IOex; VO=0; ideal switch in

the on state

If VCT < VThn or VCT > VCTex switching transistor

• An intermediary region, active forward region aF

VCT(VThn; VCTex) – permanent conduction (amplifier)

Tn

Symbols of the transistors

G - gate or grid D - drainS - source

n-type p-type

n-channel enhancement-type MOSFET

p-channel enhancement-type MOSFET

B – baseC – collector E – emitter

npn BJT pnp BJT

Controlled switch model

vCT >VCTex; T- (exc); iOT >0; vOT0

vCT <VThn; T- (off); iOT = 0; vOT=VPS

vCT >VThp ; T- (off); iOT =0; vOT=VPS

vCT <VCTex ; T- (exc) ; iOT >0 ; vOT0

The controlled switches are complementary

TpTn

Switching Transistor

MOSFET DIGITAL CIRCUITS

➢ Ideal controlled switch model

➢ Implementation with MOSFET

Logic inverter

0V logic 0

VPS logic 1

vA Tn vy

VCoL = 0 (off) VPS

VCoH = VPS (on) 0

logic 0 - 0V

logic 1 - VPS

Tn and R Tp and R

Logic inverter

Critical analysis of the logic inverter

With Tn and R

Disadvantage elimination:

R as small as possible, ideal R→0;

Critical analysis of the inverter

withTn and R

Solution: replacement of R with a controlled switch

Disadvantage elimination:

R as small as possible, ideal R→0; R as large as possible, ideal R→∞

Two possible solutions:✓ complementary switches

✓ same input

TTLCMOS

✓ complementary inputs

✓ same switch

MOS transitors BJT transitors and R

CMOS Logic Inverter

vGSn=vI

vGSp=vI-VDD

Logic function NAND, NOR

a) What is the truth table?b) What are the states of thetransistors?c) How can we eliminate the disadvantages due to the presence of R ?

Problems

Logic function NAND, NOR

CMOS NAND

a) What is the truth table?b) What are the states of alltransistors?

Problem

CMOS NAND

CMOS NOR CMOS AND

20

Transfer characteristicof the CMOS inverter

Ideal (input) Real

Noise margins

maxmax

minmin

OLILL

IHOHH

VVNM

VVNM

Voltage levels and

noise margins for

CMOS logic family

supplied at +5V

V1V5.0V5.1 LNM

V1V5.3V5.4 HNM

BJT DIGITAL CIRCUITS

➢ RTL technology

➢ TTL technology

Switching BJT, digitalcircuit

Resistor to limit the base current

B

Co

B

BECoB

R

v

R

Vvi

7.0

Cex

Bsat

ii

C

Al

C

CEsatAlCex

R

V

R

vVi

2.0

T – (off), if vCo< 0.6V

T – (on), if iB > iBsat

Problem

V5;150

;k2;k100

Al

CB

V

RR

a) What is the state of T if vCo=0V?

What is the vCE value?

b) What is the state of T if vCo=5V?

What is the vCE value?

c) What is the vCo range to keep T off?

What about to keep it in saturation?

d) If 0V => logic 0 and 5V => logic 1,

what is the logic function of the

circuit if ]V}[5;0{Cov

Bipolar Digital Circuits

• Inverter RTL Technology

• NOR

a) What is the schematic for the 2-input NAND gate in a RTL technology?b) What is the truth table?c) What are the states of the transistors?

Problem

TTL: Transistor-Transistor Logic

• Logic Inverter

• from technological reasons in TTL integrated circuits the use of only npn type transistors is preferred

• identical transistors but complementary control

vI1, vI2 ?

Standard TTL gate OPTIONAL

Voltage levels and noise margins for TTL family

Supply only with 5V

TTL sub-families with improved performances

Schottky transistor Schottky diode: a metal to semiconductor junction, in conduction 0,5V

vBE=0,8V; vBC=0,5V; vCE=0,3V

• Schottky transistor doesn't enter saturation mode

• increases the commutation speed

Simplified structure of a 2-input NAND gate from the Low-power Schottky family

Analog Switch (AS)

The AS is an electronic circuit that allows or blocks an input signal to pass to the output of the circuit according to a control signal.

vCo - two levels: VCoL<VTh and VCoH>VCTex

vCo=VCoL ; Tn-(off) ; vO=0

vCo=VCoH ; Tn- (exc) ; vO= vI

no power consumption

VThpVThn

Bidirectional AS with two complementary

controlled switches

vCo=VCoL ; AS - (off) ;

vCo=VCoH ; AS - (on) ;

C=0; AS - off; vO=0

C=1; AS - on; vO=vI

IC 4066 –4 transmission gates; supplied at 10V, ron=150Ω

DG400 (Siliconics) ron=20Ω

VCoH=VDD; VCoL=VSS

vI (VSS; VDD)

AS

AS – CMOS implementationbidirectional transffer

Application: three-channel MUX

What is the output voltage in each of the three situations? Justify the answer.

Application: three-channel MUX